It is provided a method for optimizing a treatment plan for use in radiation therapy. The method is performed in a treatment planning system and comprises the steps of: obtaining a first quality function of the treatment plan, the first quality function yielding an output value based on an input treatment plan； obtaining a first pair of input values and a slope indicator, the input values associating a value of the first quality function to a value of a score； constructing a score function that maps values of the first quality function to values of the score by fitting a curve to the first pair of input values and the slope indicator； and optimizing the treatment plan with respect to the score function, by varying the treatment plan such that the value of the score function is either improved or constrained to a feasible range of score values.
